Although tumor-specific immune cells are found in stage IV breast cancer patients, they are not able to control tumor growth. This is in part due to tumor stroma that tightly surrounds tumor nests. Tumor stroma contributes to immune evasion of breast cancer in at least two critical ways; by creating a physical barrier that prevents direct contact between tumor infiltrating immune cells and malignant cells, and by producing immunosuppressive cytokines that directly block activation of immune cells and/or attract/activate immuno-suppressive cells such as regulatory T-cells. In this proposal, we will test a new stem cell gene therapy approach that targets tumor stroma cells with the goal to enable existing immune cells to control tumor growth. The specific tumor microenvironment as well as cytokines/growth factors produced by tumor cells trigger differentiation of tumor-infiltrating hematopoietic tumor cells into a unique type of macrophages (so called tumor-associated macrophages -TAMs), with a gene expression signature that is distinct from that of tissue macrophages and myeloid cells. TAMs are the prevalent stroma cell type and the number of TAMs directly correlates with breast cancer malignancy. Our central hypotheses are that i) bone marrow derived TAM progenitors can be used to deliver therapeutic transgenes to the tumor stroma, ii) the unique mRNA and microRNA expression profile of TAMs can be used to construct TAM-specific transgene expression systems, and iii) that drug controlled, transgene mediated killing of stroma cells and/or degradation of stroma protein enable long-term control of cancer. We provide a series of preliminary data that support the feasibility of our strategy. We will test our hypotheses in a mouse model of breast cancer involving rat neu-transgenic mice (neu-tg) and syngeneic mammary carcinoma cells (MMC). Key findings will be validated in a second breast cancer model that involves 4T1 cells and BALB/c mice. PUBLIC HEALTH RELEVANCE: Among the tumor stoma cells, tumor-associated macrophages (TAMs) are the prevalent cell type and the number of TAMs directly correlates with breast cancer malignancy. TAMs are a macrophage population that is phenotypically different from other tissue macrophages and bone marrow cells. Our central hypothesis is that bone marrow derived TAM progenitors can be used to deliver therapeutic transgenes to the tumor stroma. Specifically, in a mouse model of breast cancer, we plan to genetically modify, ex vivo, bone marrow stem cells and transplant them into conditioned recipients where they engraft and provide a continuous source of TAM progenitor cells, thereby enabling long-term control of cancer. In this proposal we will focus on transgenes that eliminate obstacles to naturally existing anti-tumor immune cells, created by tumor stroma.